1. Field of the Invention
This invention relates to a thermal fuser and an image formation apparatus.
2. Description of the Related Art
A thermal fuser of an image formation apparatus comprises an excessive temperature rising prevention device such as a thermostat, a temperature fuse, or the like in series with a heating source to prevent a heating roll from being excessively heated (excessive temperature rising) because of a failure of a temperature sensing member or a failure of a controller for controlling turning on/off power supplied to the heating source or voltage.
JP-B-Hei.4-39077 and JP-B-Hei.4-77313 disclose arts wherein when temperature excessively rises, a bearing or a frame for supporting a heating roll softens, whereby the heating roll is brought closely to an excessive temperature rising prevention device for preventing a rapid excessive temperature rising. The arts involve the following problem: If the heating roll is made thin, thermal conductivity in an axial direction of the heating roll is worsened as the heating roll is made thinner and thus the temperature of the bearing, etc., supporting the heating roll is hard to rise and the bearing, etc., is hard to soften and therefore an intended result cannot be produced.
JP-B-Hei.4-77314 disclose an art wherein an excessive temperature rising prevention device is installed in an axial end part of a heating roll and when the heating roll is thermally expanded axially, the heating roll is brought closely to the excessive temperature rising prevention device, thereby preventing a rapid excessive temperature rising. The art involves the following problem: As the heating roll is made thinner, thermal conductivity in the axial direction of the heating roll is worsened and thus when an excessive temperature rising rapidly occurs, the temperature of the end part of the heating roll is too low and the excessive temperature rising prevention device does not operate.
JP-A-Hei.5-333744 discloses an art discloses an art wherein a temperature control circuit and a circuit for shutting down power to a heater when an excessive temperature rising occurs are provided separately; the art has a problem of leading to an increase in costs.
It is common practice to use a halogen heater as a heating source of a thermal fuser of an image formation apparatus. The halogen heater is a heater provided by winding a wire material including tungsten as a main component like a coil having proper sparse and dense portions to form a filament and sealing the filament in a quartz glass column together with halogen mix gas.
In the image formation apparatus, thinning a heating roll for lessening the heat capacity of the heating roll is developed for the purposes of shortening the warming-up time at the starting time and saving energy. In recent years, a large number of heating rolls with iron as a material have been become commercially practical to compensate for the strength resulting from thinning the heating roll. Thinning the heating roll and selecting iron as the material cause the thermal conductivity of the heating roll to be lowered and temperature unevenness in the axial direction of the heating roll to be increased.
The art of thinning the heating roll of the image formation apparatus to lessen the heat capacity of the heating roll is developed for shortening the warming-up time and saving energy. As the heat capacity of the heating roll is lessened, a temperature rising of the heating roll when an abnormal temperature rising occurs becomes too large and the apparatus easily falls in a dangerous state of catching fire, etc., before the excessive temperature rising prevention device operates; this is a problem.
As means for preventing this, the excessive temperature rising prevention device may be brought near to the heating roll or the operating temperature of the excessive temperature rising prevention device may be lowered, thereby hastening the operation of the excessive temperature rising prevention device when an abnormal temperature rising occurs. In either case, however, it is feared that the excessive temperature rising prevention device may malfunction during the operation of print, etc., making it impossible to normally use the apparatus; this is a problem.
As one problem involved when axial temperature unevenness occurs in the heating roll, when the relative positional relationship between the filament forming a part of the halogen heater and the temperature sensing member changes, a temperature sensing failure occurs. This is caused by the fact that the surface light distribution of the heating roll changes from one point to another depending on the sparseness and denseness of the filament. In the dense portion of the filament, the surface temperature of the heating roll becomes high as compared with its surroundings; in the sparse portion of the filament, the surface temperature of the heating roll becomes low as compared with its surroundings. Therefore, as the whole heating roll, the temperature controlled by the temperature sensing member varies depending on which of the sparse and dense portions of the filament comes to the portion in which the temperature sensing member exists. Taking variations in quality at mass production time of halogen heaters, variations caused by tolerance of the halogen heater attachment part, and the like into consideration, this is not a negligible value.
Another problem is an excessive rising in the temperature of a portion through which no paper passes when narrow paper is printed. To prevent this, an art of printing while selectively changing a plurality of halogen heaters different in light distribution has been known. Even in the art, however, when the halogen heaters are changed, a fixing failure or hot offset may occur depending on the rotation direction of the heating roll and the relative positional relationship between the temperature sensing member and the halogen heater, and therefore this leads to a problem on image quality.
It is an object of the invention to solve the above-described problems by devising placement of a heating source to thin a heating roll for lessening the heat capacity of the heating roll and provide an excellent thermal fuser and an excellent image formation apparatus without producing any disadvantage by simple improvement in the configuration.
To accomplish the object, according to a first aspect of the invention, there is provided a thermal fuser comprising: a heating source; a heating roll containing the heating source; a pressure member disposed to press-contact with the heating roll; a temperature sensing member for sensing temperature of the heating roll; a temperature controller for controlling electric power supplied to the heating source based on the temperature sensed by the temperature sensing member; and an excessive temperature rising prevention device disposed in the proximity of the heating roll and connected in series to the heating source, wherein the heating sources is disposed at a position close to the excessive temperature rising prevention device from the center of the heating roll.
In this case, the thermal fuser is characterized by the fact that the heating source has bend parts in the proximity of both ends thereof; a portion of the heating source through which paper passes is made eccentric; and the eccentric portion is disposed to be close to the excessive temperature rising prevention device side from the center of the heating roll. In the thermal fuser, it is preferable that the heating source is a plurality of heating sources. The heating value of the heating source disposed to be the closer to the excessive temperature rising prevention device from the center of the heating roll, is larger. In the thermal fuser, it is preferable that a heating source with a wider effective heating range is disposed more downstream in a rotation direction of the heating roll viewed from the excessive temperature rising prevention device. In the thermal fuser, it is preferable that a flow passage shape for making the amount of cooling air passing through the proximity of the excessive temperature rising prevention device larger than that through any other portion is provided.
Next, according to a second aspect of the invention, there is provided a thermal fuser comprising: a heating roll containing a halogen heater; a pressure member disposed to press-contact with the heating roll; a temperature sensing member for sensing temperature of the heating roll; and a temperature controller for controlling electric power supplied to the halogen heater based on the temperature sensed by the temperature sensing member, wherein the thinnest portion of the heating roll in an area through which paper passes is not more than 0.5 mm; and a part of the halogen heater corresponding to a position of the temperature sensing member is positioned at one of a top and a bottom of a local light distribution ripple in an axial direction of the halogen heater.
According to a third aspect of the invention, the thermal fuser according to the second aspect of the invention, wherein the halogen heater is a plurality of halogen heaters different in light distribution; the halogen heaters are changed in response to print conditions to conduct fixing; for the halogen heater upstream in a rotation direction of the heating roll viewed from the temperature sensing member, the bottom of the local light distribution ripple in the axial direction is placed at the position corresponding to the temperature sensing member; and for the halogen heater downstream in the rotation direction of said heating roll viewed from said temperature sensing member, the top of the local light distribution ripple in the axial direction is placed at the position corresponding to the temperature sensing member.
Further, according to a fourth aspect of the invention, there is provided a thermal fuser comprising: a heating roll containing a heating source having difference in light emission amount in a length direction, a pressure member disposed to press-contact with the heating roll; a temperature sensing member for sensing temperature of the heating roll; and a temperature controller for controlling electric power supplied to the heating source based on the temperature sensed by the temperature sensing member, wherein the thickness of an area of the heating roll through which paper passes is thin; and a part of the heating source corresponding to a position of the temperature sensing member is positioned at a part where a temperature gradient of the heating source is moderate.
In the fourth aspect of the invention, the thickness of the area through which paper passes is may be to such an extent that a temperature gradient appears based on the light emission amount difference of the heating source in a length direction of the heating roll in the proximity of the part where the temperature sensing member is placed, and the thinnest portion in the area through which paper passes may be not more than 0.5 mm. Further, preferably the heating source is a heater comprising light emitting parts and non-light emitting parts placed alternately in the length direction of the heating source and the part where the temperature gradient is moderate is a portion corresponding to one of the light emitting part and the non-light emitting part.
Further, according to the invention, there is provided an image formation apparatus comprising any of the thermal fusers described above.
The image forming apparatus of the invention comprises a sequence controller for shutting down power supply of a heating source when an image formation process is stopped midway, and then stopping rotation of a heating roll after the expiration of a setup time interval, whereby providing a still thinner heating roll is facilitated.